1. Field of the Invention
This invention relates to a solid electrolytic capacitor with a conductive polymer layer as solid electrolytic layer.
2. Description of the Related Art
Solid electrolytic capacitors that utilize a solid electrolytic layer are widely used in electronic devices because, in addition to their small size and large storage capacity, their equivalent series resistance is small. Especially, solid electrolytic capacitors that utilize a conductive polymer layer as the solid electrolytic layer have low equivalent series resistance compared to the ones that utilize manganese dioxide or TCNQ complex as the solid electrolytic layer, and their production is increasing in recent years.
Solid electrolytic capacitors are usually formed by accommodating a capacitor element inside an exterior case for protection, and the shape of the exterior case is formed to be suitable for mounting onto a circuit board. FIG. 7 is a schematic diagram of a capacitor element (1) of winding type that has been conventionally used. The winding-type capacitor element (1) has a winding portion (2) that is formed by rolling a band-shaped anode foil (21), a band-shaped cathode foil (22), and a band-shaped electrically insulating separator (23) that is sandwiched between the anode foil and the cathode foil. For the anode foil (21), a foil of valve metal such as aluminum, tantalum, niobium, or titanium is used. The surface of the anode foil (21) is etched and a dielectric oxidized film is formed. For the cathode foil (22), too, the foil of the valve metal is usually used. A binding tape (24) is pasted on the lateral side of the winding portion (2) to prevent the winding portion from collapsing. Tab terminals (31) and (32), made of aluminum, are respectively connected to the anode foil (21) and the cathode foil (22). An anode lead wire (41) is electrically connected to the anode foil (21) and a cathode lead wire (42) is electrically connected to the cathode foil (22) through the tab terminals (31) and (32).
FIG. 8 is a cross-sectional view of a vertical chip-type solid electrolytic capacitor that uses the capacitor element (1) described above. In this view, the cross section of the capacitor element (1) is simplified. In the capacitor element (1), the solid electrolytic layer is formed between the anode foil (21) and the cathode foil (22), and the capacitor element (1) is then accommodated in a metal cylindrical container (11). The metal container (11) is made of aluminum, for example. A seal rubber (12) is arranged so as to close the opening of the metal container (11). The seal rubber (12) is made of butyl rubber, for example. It is ensured that the opening is sealed and the seal rubber (12) is securely fastened by clamping the side wall of the metal container (11) and bending the tip of the metal container (11) inward, forming as a curled portion (13).
A lid-shaped floor board (14) is fastened on one end of the metal container (11). The floor board (14) is made of an insulating plastic, for example. The anode lead wire (41) and the cathode lead wire (42) both penetrate the seal rubber (12) and the floor board (14). In the places where these lead wires (41) and (42) protrude from the floor board (14), they are plastically formed to respectively make an anode electrode terminal (61) and a cathode electrode terminal (62). The electrodes (61) and (62) have a flat plate shape and are disposed on the surface of the floor board (14).
Solid electrolytic capacitors are used in various electronic devices and are installed on the circuit boards of these electronic devices. For the electronic devices that are steadily becoming smaller and thinner such as notebook PCs and mobile information terminals (PDA), there is a need for denser and thinner circuit boards, and accordingly, there is also a need for smaller and thinner solid electrolytic capacitors. Therefore, to fulfill this need, it is conceivable to make the thickness of the seal rubber (12) and the floor board (14) of the conventional solid electrolytic capacitor smaller. It should be noted that it is difficult to make the size of the capacitor element (1) itself smaller. Because the shape of the capacitor element (1) is directly related to the electrical properties of the solid electrolytic capacitor, it is difficult to change the shape while still keeping predetermined electrical properties.
However, for the above-described solid electrolytic capacitor, the seal rubber (12) needs to have a thickness of a certain value or more to prevent the moisture of the atmosphere from reaching the capacitor element (1). The floor board (14) too needs to have a thickness of a certain value or more to obtain a desired durability to withstand an impact. Therefore, there is a limit in trying to make the solid electrolytic capacitor smaller and thinner by reducing the thickness of the seal rubber (12) and the floor board (14). In addition, the curled portion (13) must be formed in the metal container (11) to fasten the seal rubber (12) in the metal container (11). The height of the solid electrolytic capacitor increases by the height of the curled portion (13).
As the lengths of the lead wires (41) and (42) from the winding portion (2) to the electrode terminals (61) and (62) become shorter, the equivalent series resistance of the solid electrolytic capacitor becomes smaller. It is however not possible to shorten the lead wires (41) and (42) to lower the equivalent series resistance, because the seal rubber (12) and the floor board (14) need to have at least a certain thickness. It is desirable to minimize the contribution that these lead wires (41) and (42) have on the equivalent series resistance, because the advantage of the solid electrolytic capacitor is small equivalent series resistance.
The present invention was made in order to solve these problems and to provide a smaller and thinner solid electrolytic capacitor in which the contribution on the equivalent serial resistance by the lead wires is small.